The FBI has ordered more U.S. terror suspects be put under 24/7 surveillance in the wake of the Garland, Texas shooting and a renewed emphasis by ISIS and other terror groups for potential American recruits to launch attacks at home, according to three FBI officials.

The officials told ABC News that agents have been ordered to review the cases of so-called “marginal” or “borderline” suspects, terms that had been applied to one of the gunmen in the Texas attack, Elton Simpson of Phoenix. FBI agents were familiar with Simpson and the views he espoused, but he was not put under 24/7 surveillance. He was viewed as being “more talk than action,” one agent said.

“We do not want to risk another marginal, homegrown extremist who was viewed as dangerous going active,” said one of the FBI officials. All three FBI officials spoke on the condition of anonymity because they are not authorized to speak publicly.

Agents are now reviewing cases and “gauging” how many more targets should be placed under 24/7 surveillance.

Today FBI Director James Comey and Department of Homeland SecuritySecretary Jeh Johnson held a video conference call with “federal, state and local law enforcement partners” to discuss the Texas attack and the “current threat environment,” a spokesperson for the FBI said. The spokesperson said such conference calls were “commonplace and assist in the timely and comprehensive information sharing needed to address the persistent and pervasive threats fueled by social media.”

But the officials who spoke to ABC News described a “panic” and “crisis” inside the FBI because the agency and the rest of the nation’s homeland security infrastructure are not built to deal with the non-stop flow of homegrown extremists and possible threats that mark the current environment within the U.S. Another FBI spokesperson did not respond to request for comment on the broader concerns at the Bureau.

Another senior FBI official who was on the call with Comey and Johnson told ABC News of the domestic threat, “Every city has a subject or subjects of concern.”

Speaking to reporters, Comey and senior leaders did not characterize the atmosphere as either “crisis” or a “panic” but acknowledged that the bureau if facing serious “challenges” because the system is stretched thin.

The officials said the FBI is increasingly seeing online ISIS supporters and recruiters urge Americans to direct their efforts to target their own country if they can’t travel to Syria or Iraq to join the Islamic State.

In one Twitter message to Simpson from an ISIS recruiter known as “Miski” prior to the Texas shooting, Simpson was told he could more easily meet Allah by staying in the U.S. than by “touring,” or traveling to Syria.

Comey told reporters Thursday that the messaging shift has been detected in recent weeks by American analysts and it’s troubling.

“If you can’t travel, kill where you are,” Comey said, explaining the new directives coming from ISIS recruiters.

ABC News contributor Steve Gomez, the former head of counter-terror investigations at the FBI’s Los Angeles division, said, “This upends the entire U.S. counter-terror protocol, which has always called for the U.S. to fight terror in the Mideast to keep it from winding up inside the homeland.”

More than a hundred U.S. residents are considered possible terror threats, the FBI announced recently.

The number of “borderline” or “marginal” suspects is substantially larger and would badly strain FBI resources if they required 24/7 surveillance by agents. Typically, it takes at least 30 agents for a full, round-the-clock surveillance of just one suspect.

Comey said that Simpson was first placed under FBI watch since 2006, when the agency first learned the Phoenix-area man wanted to join al-Shabab, the al Qaeda-linked group in Somalia. Simpson was ultimately indicted on terrorism charges and convicted, but due to questions over the government’s case, he never went to prison and was sentenced to probation. The FBI officially closed its case into Simpson last year.

Two months ago, however, the FBI found out about Simpson’s newer social media postings, suggesting “renewed interest in jihad” with ISIS. Agents then tried to determine “what he was up to,” the director said. Just hours before the controversial gathering in Garland Sunday, the FBI realized that Simpson might consider attacking the site.

About three hours before the event was to begin, the FBI sent Garland police an intelligence bulletin warning that Simpson may be interested in traveling to Texas and attacking the event, according to Comey, who said the bulletin included a picture of Simpson and a suspected license plate.

Prior to that, Comey insisted the FBI had no reason to believe Simpson would actually try to carry out an attack.

The incident is the starkest example to date showing the tenuous position American counter-terror officials now find themselves in.

“It’s back to the unknown, like post 9/11,” said one senior FBI officials. “Are we really one step ahead of it? The threat has evolved in the just last six months.”

In addition to the fear that people perceived as “marginal” extremists could act, the FBI is being “overwhelmed” by the exponential growth of recruitment activities in cyberspace.

“The problem is there are so many new platforms and new media forums it’s trying to catch up on that,” said one of the FBI officials. “The threats are moving a hundred miles an hour and the difficulty is differentiating between kids who are just tweeting and guys that are for real.”

ABC News’ Pierre Thomas contributed to this report, along with Michele McPhee, a Boston-based freelance reporter and frequent contributor to ABC News.